Whale slaughter takes heavy toll on sea life

By MICHAEL McCARTHY

The wildlife of the north Pacific has been devastated in a 50-year chain reaction set off by commercial whaling, scientists claim.

The number of sea lions, sea otters and several species of seals in the area has slumped because of the slaughter of more than 500,000 whales in the Pacific between 1949 and 1969, the American marine biologists say.

As the great whales such as sperm, fin and sei whales disappeared, killer whales, which used to prey on them extensively, were forced to find new food sources.

They switched to the smaller marine mammals, dramatically reducing their numbers.

Harbour seals were the first affected, followed by fur seals, then sea lions and most recently sea otters. Their alarming declines had been a mystery.

The scientists, whose theory is in the Journal of the US National Academy of Sciences, say a "domino effect" set off by the huge post-war whaling boom, led by Russian and Japanese whalers, is the cause.

If they are right, it is one of the most chilling examples of how man's large-scale interference with ecosystems can have unintended and terrible consequences elsewhere.

Along the Alaskan coast and in other parts of the north Pacific and the Bering Sea, populations of Steller's sea lions, seals and sea otters have all fallen "precipitously".

Earlier theories suggested that climate change and/or industrial fishing activities were to blame by reducing the animals' fish-food.

But the number of seabirds that prey on the same fish species has not dropped, the researchers say, and the surviving animals are not undernourished.

They suggest that predation by killer whales, or orcas, has driven their numbers down.

Killer whales feed on great whales - their name was originally "whale killers" before being transposed - but will eat other marine mammals if necessary.

Modern industrial whaling took place in the North Pacific only from the late 1940s, as first Japanese then Russian whalers moved from their depleted home grounds. Then the slaughter started in earnest.

"In waters within 200 nautical miles of the Aleutian islands and north coastal Gulf of Alaska alone, a minimum of 62,858 whales and an estimated 1.8m tons of whale biomass were taken between 1949 and 1969," the scientists report.

"As a measure of the magnitude of change in whale abundance in this region over this time, only 156 whales were harvested there after 1969.

"Altogether, at least half a million great whales were removed from the north Pacific Ocean and north Bering Sea during this period.

"By the mid-1970s, all great whale stocks in the north Pacific Ocean were severely diminished."

Although some species had recovered, total whale stocks were thought to be only 14 per cent of pre-exploitation levels.

The subsequent decline of the smaller marine mammal populations was consistent with the expectation that the killer whales would have to switch to other food sources. To test their theory, the researchers compared the nutritional requirements of killer whales, the nutritional value of sea lions and otters, and the number of deaths required to account for the decline in sea lion and sea otter numbers.

They found a shift of less than 1 per cent of the whales' daily calorific intake was enough to eventually wipe out seal lion and otter populations.

The killer whales seemed to have started eating smaller coastal marine mammals in the 1970s.

They moved from harbour seals and fur seals, which were easy to catch and full of nutrition, to the more difficult sea lions, then to the smaller sea otters.

In a further part of the domino effect, the disappearance of the sea otters led to a boom in the sea urchins which they used to eat.

The sea urchins then overgrazed the Alaskan kelp beds along the coast.

"If our hypothesis is correct, either wholly or in part, commercial whaling in the north Pacific Ocean set off one of the longest and most complex ecological chain reactions ever described," the scientists said in their report.

One of the paper's authors, Dr Jim Estes of the University of California at Santa Cruz, said that when any species was exploited to excess it could trigger a broad and devastating "domino effect".

"The ecosystem impacts are significant. What we've seen is that the kelp forest ecosystem in south-western Alaska went from being robust to being gone," he said.

"It's staggering that it occurred over such a large area in such a short time, just a few years.

"The ... interconnectivity, that urchin explosions could be linked to whaling 50 years ago, is amazing."

- INDEPENDENT